Can I set the default font in Outlook?

Yes this is possible with Policy Patrol Signatures. Policy Patrol Signatures works at Outlook level and is capable of configuring the default font size for Outlook clients.

For Policy Patrol Signatures you need to install software on the Outlook clients (you can use a group policy for that) and then you can configure the default font size centrally for all clients. Further instructions can be found in the Policy Patrol Signatures manual.